Method and apparatus for depositing particles on surfaces

ABSTRACT

A method of providing particle deposition on a semiconductor wafer or other surface first provides a flow of clean gas into a deposition chamber that purges the chamber prior to introduction of the wafer, and after introduction, continues the flow of clean gas. An aerosol is mixed into the clean gas flow for a desired length of time, so that as the combined flow passes through the deposition chamber particles are deposited on the wafer supported in the chamber. After the deposition has continued for either a desired particle count or a length of time, the flow of aerosol is discontinued, and a clean gas flow sheath is provided over the wafer as it is removed from the chamber. The apparatus carries out this method by providing a source of a clean gas, valves for controlling aerosol introduction into the clean gas, and a support for the wafer in the path of gas introduced into the chamber.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for depositingstandard or known size particles on a semiconductor wafer or othersurface without mechanical movement of the wafer during the depositionprocess while precisely controlling the depositing of particles.

In the semiconductor industry, it is often necessary to determine thenumber of particles on a wafer by use of a wafer inspection tool. Suchtools generally operate on the principle of light scattering, whichmeasures the particle size on the wafer by the amount of light scatteredby each particle as a laser beam is swept across the wafer surface. Tostandardize such an inspection tool it is necessary to deposit particlesof a known size on a wafer and place the wafer in the inspection tool todetermine the response from that reference wafer particles, forcomparison with known factory settings of the inspection tool. It isalso important to know the number of particles on the wafer, and to havea reasonably uniform distribution of particles on the wafer surface.This will essentially be a calibration process to ensure that theresults obtained from the wafer inspection tool are accurate or in someway can be correlated to standard readings.

In addition, the original calibration of the inspection tool in thefactory is also based on the use of known size particles on a wafer. Themost commonly used calibration particles are polystyrene latex (PSL)spheres, usually in the size range between 0.1 μm and 2 μm. Also, thereis a need to deposit uniform size particles of silicon, silicon dioxide,silicon nitride and similar materials on wafers, to determine theresponse of the inspection tool to particles of a different material.See for example the article by S. A. Chae, H. S. Lee and B. Y. H. Liu"Size Response Characteristics Of A Wafer Surface Scanner for Non-Ideal,Real-World Particles" Journal of the IES 35(6):45-52, December 1992.

A commonly used method of depositing such standard calibration particleson a wafer is to make a suspension of the particles, such as PSL, inwater, and then atomize the suspension to form a spray. The spray isallowed to dry leaving uniform sized PSL particles suspended in air toform an aerosol. The aerosol is then introduced into a chamber thatholds the wafer to permit these particles to deposit on the wafersurface. The mechanism that causes particles to deposit on a wafer froman aerosol are well known. Theoretical and experimental studies haveshown that the principle deposition mechanisms are gravitationalsettling, and diffusion. If the particles are electrically charged,electrostatic attraction also plays a role in particle deposition. Seethe article by B. Y. H. Liu and K. H. Ahn, entitled "Particle Depositionon Semiconductor Wafers," Aerosol Sci. Technol. 6:215-224 1987 and thearticle by D. Y. H. Pui, et al. "Experimental Study of ParticleDeposition on Semiconductor Wafers," Aerosol Science and Technology12:795-804, 1990.

To carry out the deposition in a chamber containing a wafer by thesimple introduction of an aerosol into the chamber has a number ofdisadvantages. First, when the wafer is placed in the chamber, theuncontrolled contaminate particles already in the chamber can deposit onthe wafer to cause unwanted contamination of the wafer. Second, when theaerosol is first introduced into the chamber, the particle concentrationin the chamber will build up slowly as the aerosol is mixed with therelatively clean chamber air. The rate of particle deposition,therefore, will vary as a function of time due to the varying particleconcentration. The varying particle concentration makes it difficult tocontrol the number of particles deposited on the wafer. Similarly, whenthe aerosol flow into the chamber is stopped, the particle concentrationin the chamber will slow decay, again causing variations in particledeposition rate with time and making it difficult to control or know thenumber of particles deposited.

U.S. Pat. No. 5,194,297 entitled "SYSTEM AND METHOD FOR ACCURATELYDEPOSITING PARTICLES ON A SURFACE" by Bradley H. Scheer, et al.discloses a method of attempting to overcome such difficulty byproviding a clean sheath flow area in the chamber. That is, there is aregion where there is clean air providing a sheath over the wafer, and aseparate aerosol flow area is provided in the same deposition chamber.The wafer is first introduced into the clean sheath flow area prior todeposition, and is protected from deposition of particles when in theclean sheath flow area. The wafer is then moved mechanically into theaerosol flow area for deposition, and then back to the clean sheath flowarea following deposition, to complete the cycle. Mechanical movement ofthe wafer in a deposition chamber is complicated, and can cause unwantedcontaminate particles to be generated by the mechanical movement of aconveyor belt, a robot arm or other gear mechanism needed to move thewafer between the two regions within the chamber.

The present invention seeks to overcome the problems inherent inaccurately depositing known size particles onto a wafer, withoutintroducing unwanted contaminates, in a simple, easily controlledmethod.

SUMMARY OF THE INVENTION

The present invention, in its simplest form, provides a chamber in whicha wafer can be supported in a known manner, and initially provides asource of particle free gas, such as air or dry nitrogen, to purge thechamber of unwanted particles, making the chamber clean and particlefree. After purging, a wafer is placed into the chamber, and an atomizerflow carrying known size particles is combined with the clean air flowon the inlet of the chamber. Thus there is a mixing of the aerosolcontaining the known size particles and the clean air flow that isintroduced into the chamber. The aerosol stream mixed with the clean airflow impinges the wafer surface in the chamber to cause particledeposition. The exhaust air flow from the chamber is usually filteredthrough a high efficiency filter to prevent the discharge of aerosolsinto the ambient clean room environment.

After a predetermined time of particle deposition, the atomizer isturned off, utilizing valves that can be manually controlled or throughan electronic timer or a computer so that the aerosol flow stops, andthe air or gas flow entering the chamber again is restored to itsoriginal clean flow. The clean air flow impinges directly on the waferand thus provides a protective clean air layer to prevent furtherparticle deposition on the wafer surface.

The aerosol flow can be turned on and off quickly, so that the number ofparticles deposited on the wafer can be accurately controlled. Theatomizer forming the particle source can be turned off when particlesare not to be deposited, which conserves such materials and theattendant cost.

In a modified form of the invention, the total flow into the chamber ismaintained at a known level, even after the aerosol flow has been addedto the clean gas flow on the inlet side of the chamber. This makes for aless turbulent air flow in the chamber. Additionally, an electrostaticcharge can be applied to the particles, to improve the performance ofthe particle deposition system.

A very precise control over the particle size can be achieved by addingan electrical aerosol switch which will deflect particles of the desireddiameter to the vicinity of a slit opening near the bottom of the switchhousing, and then sweeping these particles out of the switch and intothe deposition chamber.

The electrical aerosol switch in and of itself is known, but combinedwith the clean air flow and switching the known size particles into orout of a steady air stream of clean flow provides for a very precisedeposition time control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a basic particle depositionsystem made according to the present invention;

FIG. 2 is a modified form of the system of FIG. 1 illustrating a flowcontrol device for input air flow to a deposition chamber;

FIG. 3 is a further modified form of the invention providingelectrostatic charge to aerosol particles being deposited on a wafer;and

FIG. 4 shows a deposition system made according to the present inventionwith an electrical particle switch on the input side of the depositionchamber to precisely control the timing of the introduction of particlesinto a deposition chamber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a basic system for carrying out the presentinvention is shown. The deposition apparatus illustrated generally at 10is used in connection with a known deposition chamber 11 which has asupport 12 therein for supporting a wafer or object 14 on whichparticles are to be deposited. The chamber construction is well knownand normally will be in a clean room. This depositing arrangement is toprovide a known number of particles (within reasonable limits), of aknown size and density on a wafer that can be used for calibrationpurposes or for comparison with wafers in use to determine the particledistribution on such wafers. Various forms of the chamber can beutilized. The deposition apparatus 10 further includes a source of cleanair flow indicated generally at 16. It is understood that the source caninclude other gases, as well as air, such as dry nitrogen. If desired, aflow controller 18 can be put on the outlet line of the clean air flowsource 16, to provide a flow of air along a line 20 indicated at Q₁.This provides a known flow of clean gas or air, which does not have anycontaminating particles.

An atomizer indicated at 22 is used for providing known size particles,such as the PCL particle previously mentioned. The atomizer provides anaerosol, when compressed air is directed into the atomizer 22 through asolenoid valve 24 from a source of clean compressed air 26. The solenoidvalve 24 can be controlled by a control signal represented at 25provided by an electronic timer or a computer 28. The timer or computer28 also can control provision of the clean air flow from source 16 byoperating a suitable valve.

The outlet line 30 from the atomizer 22 is connected to the line 20,downstream from the flow controller 18, but between the flow controllerand the deposition chamber 11. An atomizer flow Q₂ is provided on line30. As can be seen, the line 20 has an inlet connection into theinterior of the deposition chamber 11. When the valve 24 is energized byproviding a control signal 25 from the timer or the computer 28, theatomizer 22 will discharge a known volume of air bearing particles thathave been generated in a known manner to ensure an adequate aerosol flowQ₂. The flow along the line 20 between the junction with line 30 and thedeposition chamber 11 in this form of the invention will have a totalflow of Q₁ +Q₂, and will be carrying particles into the chamber. Theseparticles will be directed as indicated at 32 on the interior chamberdown onto the upwardly facing surface of the wafer 14.

Undeposited particles will be exhausted out through line 34 through anexhaust and filter arrangement 36.

In the process of operating the apparatus shown in FIG. 1, air flow fromsource 16 is first turned on, and again it is a source of particle freegas or air obtained in a known manner. The clean air flow into thedeposition chamber 11 is permitted to purge the chamber of unwantedparticles, until it is known that the chamber is clean and particlefree. These unwanted particles will be discharged out through theexhaust line 34 and the exhaust and filter arrangement 36. Sufficientinitial clean air flow is provided for a sufficient length of time toensure that the unwanted particles in the chamber 11 have been purged.

Then, wafer 14 is placed in the chamber on support 12 (this can be anysuitable support and is shown on schematically). The clean air flow cancontinue during the installation of the wafer to keep the chamber volumefree of particles. The solenoid valve 24 is energized after the wafer isinstalled and the chamber closed, which starts the aerosol flow Q₂ ofknown size and volume of particles. The flow Q₂ enters the line 20 andmixes with the clean air flow Q₁ to form a combined aerosol stream Q₁+Q₂ that is passed into the chamber 11, and is directed toward theupward surface of the wafer 14 on which the particles are to bedeposited. The aerosol stream impinges on the wafer in the chamber 11 tocause the particle deposition on the chamber 11. The exhaust air flow isthrough a filter, which is preferably a high efficiency filter, toprevent the discharge of the aerosol into the ambient clean roomenvironment in which these processes normally take place.

After a predetermined time period of particle deposition the atomizer 22is turned off by turning off valve 24, either manually or with anelectrical signal from a precision timer or computer 28. The aerosolflow Q₁ will stop and flow entering the chamber 11 is then againrestored to its original clean air or gas flow Q₁. Since the clean airflow is impinging directly on the wafer 14, as shown, it provides aprotective clean air layer to prevent further particle deposition on thewafer surface from particles that may remain in the chamber 11 at leastfor a short period of time. When the wafer 14 has been left for asufficient period of time to ensure that a clean air layer is flowing onthe wafer, it can be removed from the deposition chamber 11 for use inan analysis instrument, or for any other use that may be desired. Thewafer does not have to be shifted from its original position within thechamber 11, and yet receives a direct impingement of clean sheath airwhile it is being placed into the chamber. The wafer will be in a purgedclean, particle free chamber to prevent contaminants from depositingthereon. After the deposition time, which can be relatively accuratelycontrolled, the wafer 14 can be removed and then will have a knownnumber of particles of a known size on it.

The aerosol flow switching method disclosed has the further advantage ofhaving atomizer 22 turned on only for the short time duration when theaerosol is needed for deposition. During the remaining time the atomizer22 is left idle, to reduce the amount of liquid suspension that must beatomized to form an aerosol, leading to a savings in both material andcost. The fact that the atomizer 22 is turned on only when needed fordeposition of particles on a wafer, also minimizes the unwanted particlebuild-up on the interior surfaces of the deposition system, making itunnecessary to take the system apart for frequent cleaning.

FIG. 2 shows a modified system for obtaining a more precise flow intothe deposition chamber 11 by avoiding flow disturbances. The system ofFIG. 1 shows the flow into the chamber 11 when the atomizer 22 is turnedon, that is, the total flow into the deposition chamber 11 will be Q₁+Q₂, and when the clean air flow only is flowing into the chamber 11 theflow is only Q₁. FIG. 2 shows a way of avoiding flow disturbances causedby changing flow volumes. In FIG. 2 the same components are numbered thesame, and include the aerosol chamber 11, the support 12 on which wafer14 is mounted. The exhaust line 34 and inlet line 20 are numbered thesame.

A bleed line 40 to line 20 is connected to the line 20 at a junction or"T" fitting 41 that is downstream from a connection 42 between the line30 and the line 20. The line 30 carries the aerosol whenever thesolenoid valve 24 is energized, as previously explained.

The line 40 is connected through a second solenoid valve 44 to anexhaust arrangement 46 that also can have a filter if desired. The valve40 is energized and de-energized or controlled by a control signal 48from the electronic timer or computer 28 which synchronizes the controlsfor the control signal 25 operating the solenoid valve 24. After theatomizer 22 is turned on by energizing solenoid valve 24, the aerosolflow is mixed with the clean air flow Q₁ at junction 42, as before, andsolenoid valve 44 is opened and controlled as to its port size (it canbe a flow control valve) diverting part of the mix flow Q₁ +Q₂ atjunction 41. The diverted flow equal to Q₂, is exhausted through theexhaust arrangement 46. The combined stream is thus divided, and theportion of the line 20 downstream, and that is in direction towarddeposition chamber 11, from junction 41 will continue to have a flowvolume Q₁, but it will be an aerosol flow. The total flow into thechamber 11 is thus kept constant at Q₁ at all times.

The process is the same as before, with the deposition chamber 11 beingpurged by a clean air or gas flow from source 16 under control of timeror computer 28, and when the deposition chamber 11 has been purged sothere are no foreign particles in the chamber, the wafer 14 or otherobject on which deposition of particles is to take place is moved ontosupport 12. Then the solenoid valve 24 will be opened to provide aerosolflow Q₁ along line 30, and valve 44 will be also open, at an appropriatetime, to control the total flow coming through line 20 between junction41 and the deposition chamber 11 at a volume of Q₁ but with aerosols inthat volume.

The timing of course can again be carried out as previously explained tostop the atomizer flow at the desired time so that the deposition on thewafer will be as desired.

FIG. 3 shows a further modified form of the present invention. In thisinstance, the aerosol represented at 50 could be the aerosol between thejunction 41 and the deposition chamber 11 of FIG. 2. The aerosol wouldbe provided as previously explained after air flow for purging thedeposition chamber 11 before the wafer is put into place. After theatomizer 22 has been turned off the chamber 11A of FIG. 3 can be purgedwith clean gas.

Single aerosol particles produced by atomization are usuallyelectrically charged. Thus, it is possible to increase the rate ofparticle deposition by means of an applied electric field. In thedeposition chamber 11A of FIG. 3, a metal plate 52 is provided and itsurrounds the inlet port 53. The plate 53 is connected to a high voltagepower supply 54 to provide an electrical potential on the plate. Themetal plate 53 is mounted on an insulating support, and connected sothat it is electrically insulated from the deposition chamber 11A. Thewafer 14 itself is connected to a ground connection through a line 55,which can connect through the wafer, or to the wafer support 12, asdesired.

In this system, when the aerosol stream entering the chamber 11Aimpinges on the wafer 14 and the particles carried in the flow of theaerosol will have an electrostatic change applied, causing the rate ofparticle deposition to increase, thereby shortening the time requiredfor particle deposition to achieve a known number of particles orparticle density on the wafer 14.

The electrically charged plate 53 can be used with either of the systemsshown in FIGS. 1 and 2, to further improve particle deposition bylessening the time needed for providing adequate deposition on aparticular wafer 14.

The same procedure for deposition is used prior to the introduction ofthe aerosol into line 20, and also upon completion of the aerosoldeposition, namely the purging of the chamber initially before the wafer14 is put into place, and providing a clean gas flow over the wafer 14after the deposition has been stopped.

FIG. 4 shows a further modified form of the present invention, againwith like components being numbered the same as in FIGS. 1-3. In thisform of the invention, the deposition chamber 11 is provided with anelectrical aerosol switch on the inlet side of the chamber. In thisinstance, the clean air flow is provided from source 16 as previouslyexplained, along the line 20. The clean gas or air flow volume is Q₁, aspreviously explained as well.

The atomizer 22 is connected through a line 30 to a conventional dryer58. The flow through line 30 is controlled by the valve 24, and controlsignal 25 from the timer or computer 28 as previously explained as well.Compressed air supply 26 is provided through the valve 24. The dryer 58is of conventional design which dries the liquid from the aerosolstream, and after passing through the conventional dryer 58, theaerosol, in dry air, is passed through an ionizer 60 which provides anelectrical charge to the particles. The ionizer also is a conventionalunit used in providing aerosols at the present time. Charged particlesare discharged out through line 30A.

The electrical aerosol switch is indicated at 62. It is a cylindricalcondenser in which laminar air flow is established. There are two inputair streams. Flow Q₁ is input into the switch at a connection 64 nearthe center of the switch, and the flow Q₂, which is the input aerosolstream, is provided at an annular input junction 66 adjacent theperiphery of the electrical switch 62.

Input air connection 64 for the clean air flow discharges air into anannular chamber or plenum 68, defined by a tubular wall 67 thatsurrounds a central electrode 70 in a housing 76. The tubular wall 67also defines an annular chamber 74 inside housing 76, but to the outsideof chamber 68. The aerosol input connection 66 lead to the annularchamber 74 that surrounds the chamber 68 so that as the clean air andthe aerosol both flow downwardly as indicted by the arrows 71 and 73.The clean air flow is a core of air near the center, adjacent theelectrode 70.

The electrode 70 is an inner cylindrical body and is connected to a highvoltage power supply. The air flows Q₁ and Q₂ both flow down through theannular space between the central electrode 70 and the outer housing 76of the electrical aerosol switch and stay relatively separate. That is,the flows do not substantially intermix. By applying a voltage to theelectrode 70, which is an inner cylinder, the particles carried in flowQ₂ through the annular chamber 74 continue to move on the outside of theclean air flow, but are deflected across the clean air flow streamcoming down around the outside of the electrode 70. The aerosolparticles thus are attracted in toward the center electrode, and it ispossible, by controlling the electrical field, to deflect particles ofthe desired diameter in the vicinity of a slit opening shown at 80 downnear the bottom of the electrode 70 which is spaced slightly from anoutlet tubular cylinder 82.

The cylinder 82 surrounds the outlet opening for air flow from theelectrical aerosol switch. The particles attracted toward slit 80 areswept out of the switch by the output aerosol stream indicated at Q₄along the line 86 leading to the deposition chamber 11. Proper sizeparticles are carried with the flow into the deposition chamber 11.Excess aerosols are contained in an exhaust stream Q₃ coming from outletpassage 88 of the housing 76 into an exhaust filter arrangement 90.

The electrical aerosol switch 62 described above has been used as anelectrostatic particle classifier for producing a monodisperse aerosolfrom an aerosol that is initially polydisperse. This structure issubject of a descriptive article by the present inventor Benjamin Liu,and D. Y. H. Pui entitled "A SUBMICRON AEROSOL STANDARD AND THE PRIMARY,ABSOLUTE CALIBRATION OF THE CONDENSATION NUCLEI COUNTER" J. ColloidInterface Sci. 47:155-171, 1974. However, adapting this type of aclassifier to the input side of the deposition chamber 11 gives a greatadvantage in being able to provide a known size particle and since theflow through the narrow slit 80 of the particles is controlled by thevoltage being applied to the electrode 70, the monodisperse aerosolparticles can be switched into the deposition chamber 11 and stoppedfrom entering the deposition chamber 11 very rapidly to give a veryprecise control to the deposition of particles on the wafer 14.

The aerosol produced by the atomizer 22 in this case can be apolydisperse aerosol, and the electrical aerosol switch 62 acts as aclassifier to provide a monodisperse aerosol into the particledeposition chamber 11.

Laminar flow is achieved in the cylindrical housing for the electricalaerosol switch 62, and it is used in its traditional role as a particleclassifier. However, the additional use as an aerosol switch because ofthe narrow slit opening that permits a particle to enter only when theelectrode is energized, makes it possible to maintain a steady air flowQ₄ into the deposition chamber 11 while quickly switching the particlesinto or out of the stream for precise deposition time control. In otherwords, there will always be an air flow through the gap 80 into thedeposition chamber 11 when the system is operating, but particles willonly be entering that gap when the electrode 70 is on.

In FIG. 4 a particle counter 94 can be provided through a line 96 as anoption which can be incorporated into the system when desired to countthe particles coming in the line 86 and thus into the deposition chamber11.

The electrical switch 62 also can be used strictly as a particleclassifier by controlling the stream of the aerosols through operationof the valve 24 as previously explained while maintaining a steady highvoltage on the electrode 70, so that laminar flow in the cylindricalhousing 76 will still be maintained, but the timing of particleintroduction would then be controlled by the valve 24 operating atomizer22 inasmuch as the electrode 70 would then be continuously energized.

The output of the airborne particle counter can be used as a signal forthe automatic adjustment of the deposition time through the timer orcomputer 28 as desired.

Automatic adjustment of the deposition time can be determined by theparticle count of particles entering the deposition chamber 11, fordeposition onto the wafer 14. An electronic controller also can be usedand controlled by the particle counter output.

The optional airborne particle counter 94 also can be used with thesystem shown in FIGS. 1 and 2 to provide automatic particle depositioncontrol.

The system thus provides for the advantages of having a clean air flowthat purges the chamber to eliminate unwanted particles from the chamberprior to the time the wafer is introduced, and then controlling theparticles being introduced into the chamber to a selected range of sizesfor a precise time to deposit the necessary number of particles on thewafer surface. The particles can be shut off in response to a timer, orafter a desired particle count as shown as well.

The use of a sheath of clean air after the particles have been depositedon the wafer 14 also ensures that unwanted particles may not bedeposited on the wafer as it is being removed from the depositionchamber.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A method of depositing particles from a sourceonto a wafer supported in place at a known location in a depositionchamber comprising the steps of:providing and maintaining a clean gasflow into the deposition chamber directed onto the wafer supported inplace at the known location to purge unwanted particles from thedeposition chamber; providing a gas flow containing particles from thesource into the deposition chamber onto the wafer supported in place atthe known location for a time to deposit a desired number of particlesfrom the source onto the wafer and then discontinuing the gas flowcontaining particles from the source; and maintaining the wafer at theknown location and providing for a selected time a clean gas flow intothe chamber after discontinuing gas flow containing particles from thesource prior to moving the wafer from the known location in thedeposition chamber.
 2. The method of claim 1 including the step ofcounting the particles from the source being introduced into thedeposition chamber for controlling particle introduction.
 3. The methodof claim 1, wherein the particles from the source carry an electrostaticcharge, and an electric field is used to deposit the charged particlesin the deposition chamber.
 4. The method of claim 1 including a step ofcontrolling a total gas flow into the deposition chamber at asubstantially constant volume during the steps of the method.
 5. Amethod of depositing particles from a source onto a wafer in adeposition chamber comprising the steps of:providing a clean gas flowinto the deposition chamber directed onto the wafer to purge unwantedparticles from the deposition chamber; providing an aerosol gas flowcontaining particles from the source and adding the aerosol flow to theclean gas flow into the deposition chamber to provide a total gas flowcontaining particles into the deposition chamber and onto the wafer fora time sufficient to deposit a desired number of particles from thesource onto the wafer, and then discontinuing the aerosol gas flowcontaining particles from the source; and maintaining for a selectedtime a clean gas flow into the chamber after discontinuing the aerosolgas flow containing particles from the source prior to removal of thewafer from the deposition chamber.
 6. The method of claim 5 including astep of controlling the total gas flow into the chamber after adding theaerosol flow to maintain a volume of gas flow into the depositionchamber substantially the same as a volume of gas flow into the chamberprior to adding the aerosol flow.
 7. The method of claim 6 wherein thestep of controlling the total flow comprises bleeding off gas from aline carrying gas flow to the deposition chamber after the step ofadding the aerosol flow.
 8. The method of claim 1 including a step ofproviding a particle classifier receiving the gas flow containingparticles from the source and classifying the particles from the sourceprior to introduction into the deposition chamber.
 9. The method ofclaim 8 wherein said classifying step comprises utilizing anelectrostatic classifier having a laminar flow of two gases, onecomprising the clean gas flow, and the other comprising the flowcontaining particles, and providing an electrostatic charge on theparticles to move the particles from the flow containing particlesthrough the clean gas flow and into the deposition chamber.
 10. A methodof depositing particles from a source onto a wafer in a depositionchamber comprising the steps of:providing a clean gas flow into thedeposition chamber to purge unwanted particles from the depositionchamber; introducing a wafer into the chamber while maintaining theclean gas flow and supporting the wafer in place at a known location inthe deposition chamber; providing a gas flow containing particles fromthe source into the deposition chamber after introduction of the waferfor a time sufficient to deposit a desired number of particles from thesource onto the wafer supported in place at the known location and thendiscontinuing the gas flow containing particles from the source; andmaintaining a clean gas flow into the chamber after discontinuing gasflow containing particles from the source for a selected time prior tomoving the wafer from the known location and prior to removal of thewafer from the deposition chamber.